Ca2+ flux in retinal rod and cone outer segments: differences in Ca2+ selectivity of the cGMP-gated ion channels and Ca2+ clearance rates

Depolarization in the dark to ≥ +20 mV specifically activates the cGMP-dependent conductance in photoreceptors.

• Depolarization leads to a voltage-dependent decrease in cytoplasmic Ca2+ levels.
• The activation of cGMP-dependent conductance is faster and greater in cones than in rods.
• Differences in Ca2+ homeostasis are suggested to account for the varying responses between rod and cone cells.
• Direct measurements show that cGMP-gated channels have higher Ca2+ permeability in cones.
• Cone outer segments exhibit a higher rate of Ca2+ efflux compared to rod outer segments.
• Changes in membrane current due to light are expected to cause larger and quicker decreases in Ca2+ concentration in cones than in rods.

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